1. Field of the Invention
The present invention relates, in general, to driver circuits, and, more particularly, to an automatic calibration system and method for deriving the back electromotive force voltage by sigma-delta modulation.
2. Relevant Background
Switched power driver circuits are widely used to generate power suitable for driving loads such as motors. Switched power drivers turn on and off repetitively to supply regulated voltage or current in an efficient manner (i.e., with minimal switching loss). Switched power driver circuits are associated with driver circuits that control, for example, the magnitude (by means of the duty cycle of the on and off cycles), so as to supply a desired amount of power to a load. In a typical application, a power driver circuit is controlled by a system processor, often implemented as a microcontroller IC, that generates commands to the driver circuit. The driver circuit essentially turns on and off in a predefined sequence in response to the received commands. When on, the driver circuit supplies current to the load, and when off, the driver circuit cuts off current supply to the load.
Permanent magnet motor loads, such as VCM (Voice Coil Motor), act as a motor or a generator. As a motor, the load provides a motion as a response to a voltage or current input. Additionally, if the load is in motion, it can generate a back electromotive force voltage (VBEMF). VBEMF subtracts from the applied voltage such that the motor acceleration responds to the difference in the two voltages.
In a typical application, such as a VCM motor in a disk drive, the best performance in the head positioning servo system requires that the current in the motor be proportional to the servo controller command. This helps both the positioning of the head over the data track as well as moving the head from the track in an efficient manner.
Head position control is implemented by a servo control system. Early servo control systems for low density drives used open loop positioning using stepper motor technology. However, at higher densities closed loop solutions are required. Current disk drives, for example, obtain head position information directly from data contained on the disk surface. A track number, in the form of encoded binary data, is recorded at various locations about the disk surface and uniquely identifies each recording track on the disk. Servo position, in the form of sinusoidal burst signals staggered in position between adjacent tracks can be used to determine the position of the head with respect to a track centerline. The track number and servo burst are used to compute a position error signal (PES), which is fed into the electromechanical servo position system.
In a device driving a permanent magnet motor load such as a VCM, that is both switched mode and voltage mode, the resulting steady state output current to the load is directly proportional to the average voltage applied to the load terminals minus the VBEMF. (IA=(VAVExe2x88x92VBEMF)/RL), where RL is the resistance of the motor load. In a voltage mode driver, the average applied voltage VAVE is proportional to the input command. The VCM generates VBEMF in proportion to its velocity. Therefore, for a step of command input, the current in the actuator will decrease as the VCM increases velocity. It is therefore desirable to cancel the voltage loss due to the coil resistance to keep the current applied to the actuator, IA, proportional to the command.
Hard disks operate by having the read/write heads fly over the surface of the disk platters. However, this floating action occurs only when the platters are spinning. When the platters are not moving, the air cushion dissipates, and the heads float down to contact the surfaces of the platters. This contact occurs when the drive spins down, as soon as the platters stop spinning fast enough, and occurs again when the spindle motor is restarted, until the platters get up to speed. Each time the heads contact the surface of the platters, there is the potential for damage. In addition to friction on these sensitive components, dust can be created as the heads scrape off minute amounts of material from the platters.
Traditionally, most disk drives have operated in a Contact Start-Stop (CSS) mode, in which heads come to rest on the disk surface when the drive is turned off. During start-up, the heads slide in contact over the disk surface until the disks are spinning sufficiently fast. In order to prevent adhesion of the heads to a smooth disk surface, which could impact drive spin-up, disk surfaces have been textured (roughened) in a precision process. Texturing is performed either uniformly over the entire disk surface or locally in a specific zone at the inner diameter of the disk that is dedicated for starting and stopping. While such texturing techniques have been satisfactory in the past, today""s higher areal density designs require a level of disk surface perfection beyond the texturing needed to support CSS operation.
In mobile systems, such as laptops and the like, power saving may be achieved by shutting down a disk drive whenever it is idling. Since extending battery life is such a great concern, the rate of performing a disk drive xe2x80x9cparkxe2x80x9d in a mobile system far exceeds that of a desktop. Performing a CSS in a mobile system increases the wear and tear of the read/write head that ultimately leads to a shortening of the life span of a disk drive. Additional concerns of the mobile system are shock robustness and drive capacity. In consideration of those factors, CSS technology is no longer adequate in disk drive technology.
One possible solution in disk drive technology is the advancement of load/unload technology. In drives that use load/unload technology, a lifting mechanism removes each head from the disk surface prior to power-down and returns the heads to the disk surface only after a sufficient rotation rate has been reached on the next start-up. As a result, head-to-disk contact is significantly reduced, and disk damage from such contact is virtually eliminated.
In operation, instead of letting the heads fall down to the surface of the disk when the disk""s motor is stopped, the heads are lifted completely off the surface of the disk while the drive is still spinning, using special ramps. Only then are the disks allowed to spin down. When the power is reapplied to the spindle motor, the process is reversed: the disks spin up, and once they are going fast enough to let the heads fly without contacting the disk surface, the heads are moved off the xe2x80x9crampsxe2x80x9d and back onto the surface of the platters.
However, ramp-loading topology is susceptible to temperature induced parameter variations. The objective of the ramp-loading scheme is to be able to control the velocity of the actuator by measuring the back electromotive force voltage (VBEMF) developed in the motor and using it for velocity feedback. As such, the voltage across the winding resistance (Rmotor) 50 in the coil due to motor current must be cancelled out by means of circuit design. Unfortunately, winding resistance 50 is a function of temperature. Therefore, the circuit implemented to cancel out the winding resistance 50 must be able to overcome this challenge.
FIG. 1 shows a prior art voltage feedback measurement circuit used in a typical ramp-loading circuit. The bemf op-amp 70 extracts both the Vbemf developed in the voice-coil-motor 40 as well as the undesirable voltage drop across the winding resistance 50 in the motor, Rm. To cancel out the winding resistance 50, the voltage drop across the sense resistor 30, Rs is extracted through a sense op-amp 60 and multiplied by a K factor. The resulting product is then summed with the op-amp""s output voltage, Vbemf. This K value represents the ratio between Rs and Rm. By choosing an appropriate K value, Vbemf developed across the motor can be extracted.
As the voice-coil-motor heats up, the winding resistance also increases. The sense resistor""s resistance increases at the same time, but at a different rate then the winding resistance due to the difference in temperature and temperature coefficient. Therefore, the previously determined K factor is no longer valid and a new value must be determined.
The existing methods for finding a K value require periodic calibration and updating. This periodic updating can be performed on the ramp, during track following or on the fly. Significantly, the activities of the disk drive are interrupted during any calibration operation. Precious firmware time is traded for updating the K value. Inevitably, this results in a compromise between the read/write time and the performance of the ramp-loading circuit.
Briefly stated, the present invention involves a method and apparatus for providing a continuous auto-calibration circuit in a hard disk drive system. In one aspect, a first time duration is set, wherein the first time duration is a length of time in terms of clock counts for a calibration procedure. Then, a second time duration during the first time duration is measured, wherein the second time duration is a length of time in terms of clock counts that a counter is operational. Finally, a multiplying factor is determined by dividing the first time duration by the second time duration.